What Attic R-Value Means, and Why It Changes by Climate

R-value is resistance to heat flow, nothing more

R-value measures how well a material resists heat moving through it — higher is better insulated, and it's additive: existing insulation plus new insulation adds up (R-11 already down plus R-27 added gets you to R-38). It says nothing about air sealing, moisture, or ventilation, which are separate jobs that often matter as much as the R-value number itself.

Why the recommended number isn't the same everywhere

DOE's IECC climate zones run 1 (hot, like South Florida) through 8 (subarctic Alaska), and the recommended attic R-value rises with the zone — roughly R-30 in zone 1 up to R-60 in zones 6 through 8 — because a milder climate simply has less heat to keep in or out. Miami-Dade County, FL and Aitkin County, MN are both correctly insulated at very different R-values for exactly this reason.

What your existing insulation probably measures

3-4 inches of old fiberglass or cellulose — a common finding in older homes — measures roughly R-11 to R-13, well under any current zone's recommendation. A rough rule for checking: if you can see the tops of the ceiling joists, you're under-insulated no matter what zone you're in.

Where to go next

See the range this plays out across: North Slope Borough, AK, Richmond County, NY, Río Grande Municipio, PR.

Attic Insulation: DIY or Hire a Contractor? · State Energy Rebates for Insulation (2026 Update)